Category Five Hurricane Patricia, the strongest on record, has made landfall in Jalisco state in western Mexico, meteorologists say.
The storm is seen as "potentially catastrophic" in its impact.
Mexican authorities evacuated thousands of residents along the country's Pacific coast before the hurricane hit.
The storm is comparable to Typhoon Haiyan, which killed 6,300 people in the Philippines in 2013, the World Meteorological Organization says.
A state of emergency has been declared in three states in Patricia's path.
It made landfall in the Cuixmala area of Jalisco state, some 85km (55 miles) from the port city of Manzanillo.
The US National Hurricane Center in Miami said it hit the coast with winds of 265km/h (165mph).
Video filmed in Manzanillo shortly before the hurricane struck shows trees bending in severe wind.
The centre said that Patricia was the strongest storm it had recorded in the eastern Pacific or the Atlantic.
It has already produced the third lowest barometer reading ever recorded - a pressure of 880 millibars at the centre of the storm - the centre reported.
Within two hours of the storm making landfall, the British embassy in Mexicotweeted that the worst appeared to be over.
However, as the storm moves inland over mountains, villages could suffer landslides and flash flooding, meteorologists in Mexico say,
The government has warned that ash from the nearby Colima volcano, which has become increasingly active this year, could combine with heavy rainfall to trigger huge mudflows.
Some 400,000 people live in vulnerable areas, according to Mexico's National Disaster Fund.
The hurricane's winds were strong enough "to get a plane in the air and keep it flying", World Meteorological Organization spokeswoman Claire Nullis said.
Analysis - Katy Watson, BBC Mexico and Central America Correspondent
Mexico is used to dealing with big storms during its hurricane season but the strength of this storm is unprecedented in this part of the Pacific.
The worry is that Hurricane Patricia will leave devastation in its wake, with flash floods and landslides a concern in the coming days.
Bus loads of tourists from the resort of Puerto Vallarta have been ferried to different cities. Speaking to a couple from Texas who were in the city of Guadalajara, they told me they arrived for their holiday only to be told immediately that they were being evacuated - they got out before the worst of the weather hit but many tourists and residents remained in sheltered parts of hotels or government refuges.
No matter how prepared Mexico is, these hurricanes are unpredictable - and the consequences unknown.
The western state of Jalisco is home to the resort town of Puerto Vallarta, which appeared to have escaped the worst of the storm.
Police patrols in the resort urged people to leave the shorefront for safer areas at least three blocks inland, while loudspeakers ordered hotel residents to evacuate.
The town's airport, along with two others in the path of the storm, were closed.
Schools have been closed in Jalisco, Colima and Guerrero states.
While sheltering with his family in Manzanillo, Jacob Lozano Salazar told the BBC: "We're all safe but it's really ugly here on the coast."
Residents have also been stocking up on food and other supplies, while shop owners boarded up windows.
Heavy rains often lead to flooding in the town, said Alejandra Rodriguez, who was shopping at Wal-Mart in Manzanillo with her brother and mother.
"It ends up like an island," she said.
Mexico has to deal with tropical storms arriving from both the Pacific and Atlantic oceans at this time of year.
Some of the most powerful storms in recent years
- October 1979: Typhoon Tip - largest and most intense tropical cyclone ever recorded with wind speeds of 305km/h (190mph), killed 99 people in its path across the Pacific, mostly in Japan
- August 1980: Hurricane Allen - strongest Atlantic hurricane by wind speed, with sustained winds of 305km/h, caused nearly 300 deaths in Haiti and severe damage in the US state of Texas
- April 1991: Bangladesh cyclone known as 02B - at least 138,000 died and up to 10 million made homeless after a 6m storm surge
- October 1991: Odisha or Paradip cyclone - the strongest ever recorded in the northern Indian Ocean, killed about 10,000 people, mostly in India
- August 2005: Hurricane Katrina - killed at least 1,836 people after striking US states of Louisiana and Mississippi and was the costliest storm in history, causing $81.2bn in damage (with wind speeds of 280km/h)
- October 2005: Hurricane Wilma - most intense tropical cyclone in the Atlantic basin with wind speeds of 295km/h, killing 87 people on its path through the Caribbean and Gulf of Mexico
- November 2013: Typhoon Haiyan - the strongest storm recorded at landfall, with one-minute sustained wind speeds of 315km/h, it devastated parts of the Philippines, killing at least 6,300 people